1. Field of the Invention
The present invention relates to a compound semiconductor device, and more particularly, to a resonant tunnelling barrier (RTB) structure device having a negative differential resistance characteristic caused by a resonant tunnelling phenomenon in a superlattice structure.
2. Description of the Related Art
The negative differential resistance characteristic is an useful phenomenon which can be applied to a detector or oscillator having an ultrahigh frequency, an ultrahigh speed new functional transistor, and the like (see, for example, T. INATA et al: Excellent Negative Differential Resistance of InAlAs/InGaAs Resonant Tunnelling Barrier Structures Grown by MBE, Jpn. J. Appl. Phys. Vol. 25, No. 12 (1986), pp. 983-985). Resonant tunnelling barrier structure devices such as a resonant tunnelling barrier (RTB) diode, a resonant tunnelling bipolar transistor (RBT), and a resonant tunnelling hot electron transistor (RHET), have been proposed: For example, EP-017734-A2 (corresponding to U.S. Pat. No. 4,712,121) discloses a type of RHET.
When the resonant tunnelling barrier (RTB) structure is used in a semiconductor element (i.e., diode or transistor), the RTB must have high peak current density (Jp) of more than 1.times.10.sup.5 A/cm.sup.2 and a large peak-to-valley current density (Jp/Jv) ratio of more than 10, but good values of these properties at room temperature can not be obtained as long as the RTB structure is lattice-matched to a semiconductor substrate. The following values were obtained by lattice-matched RTB structure diodes.
Where the RTB structure is made of AlAs/GaAs/AlAs structure (wherein AlAs indicates barrier layers and GaAs indicates a well layer sandwiched between the barrier layers) lattice-matched to a GaAs substrate, EQU Jp=4.times.10.sup.4 A/cm.sup.2, Jp/Jv=3.5.
Where the RTB structure is an In.sub.0.52 Al.sub.0.48 As/In.sub.0.53 Ga.sub.0.47 As/In.sub.0.52 Al.sub.0.48 As structure lattice-matched to an InP substrate, EQU Jp=4.8.times.10.sup.4 A/cm.sup.2, Jp/Jv=5.5.
In these cases, the Jp/Jv ratios were relatively small, but when a strained layer resonant tunnelling barrier structure is made of AlAs/InGaAs/AlAs (on an InP substrate), which structure does not meet the lattice-matching condition, a high Jp/Jv ratio of 14 was obtained at room temperature. The strained layer RTB structure belongs to the strained layer superlattice structure group, and it is assumed that such a high Jp/Jv ratio is obtained because a difference (.DELTA.Ec) between the conduction band bottoms of AlAs and InGaAs is large (1.2 eV or more), which form high potential barriers. Nevertheless, there is a difference (of about 4%) between the lattice constants of AlAs and InGaAs in the strained layer RTB structure, so that a large lattice strain occurs at the interfaces therebetween and internal stresses occur in each of the AlAs layers and the InGaAs layer. Therefore, when a semiconductor device including the strained layer RTB structure is operated for a long period, problems such as a deterioration of the interfaces and disappearance of a valley peak, can occur. Namely, the device tends to have a short service life.
The coinventors (T. Inata and S. Muto) of the present application proposed a resonant tunnelling semiconductor device including an RTB structure comprising first and second barriers of (In.sub.0.52 Al.sub.0.48 As).sub.z (In.sub.0.53 Ga.sub.0.47 As).sub.1-z, (0&lt;z.ltoreq.1), and a well layer of In.sub.1-y Ga.sub.y As, (0.48.gtoreq.y.gtoreq.0.46), lattice-matched to an InP substrate (in EP-0256759-A corresponding to U.S. Ser. No. 081,340, now U.S. Pat. No. 4,825,264). In this case the barrier layers are made of a quaternary compound semiconductor InAlGaAs lattice-matched to InP.